RELATION OF SOILS TO IRRIGATION. 23 



matter in the soil. As animal life is built by vegetable 

 matter, it must eventually turn back to vegetable mat- 

 ter. Ulmic acids are acids that exude from the roots of 

 MMH> plants. We should remember that nitrogen is the 

 costliest of all plant foods, and the most difficult to re- 

 tain in the soil, and plants must have it, for it corrects 

 this humic acid in the plant as well as in the soil. The 

 ulmic acids are seldom in sufficient quantity to do harm. 

 But the humic acids when shut off from the proportions 

 of nitrogen or potash botli alkalies become too con- 

 centrated, or the dead microbes become poisonous to 

 plant life, as the great French chemist Pasteur would 

 have it. Now humic acid has the same effect both in 

 plant life and in the soil for all nature was torn off the 

 same bolt. If the soil is very wet for two or three weeks 

 :;nd is well filled with vegetable matter, although the 

 plant is overgrown, it becomes sick just as much as a 

 horse with colic. But keep the soil so the air can pen- 

 etrate it and neutralize these acids, and the more of this 

 vegetable matter the better and heavier the plant will 

 fruit. One strong point in favor of irrigation is that it 

 neutralizes these acids and brings them more surely un- 

 der the control of the scientific cultivator, so that they 

 may be more fully utilized in the structural growth of 

 the plant. 



Color and Texture. The color of soil depends 

 exclusively on its composition ; humus forming nearly a 

 black soil, while sand gives a light yellow, and iron oxide 

 produces a red color. The darker soils, other things 

 being equal, have the highest absorptive power towards 

 solar heat. -This is shown when muck is applied to the 

 surface of snow in the spring. We have often found in 

 the rich valleys of the Rocky Mountain region a dark, 

 chocolate loam interspersed here and there by deposits 

 of a lighter and more chalky nature, all being, however, 

 extremely rich in gypsum and salts that are valuable in 



